An improved technique for determining iodine concentrations by absorption-edge transmission measurements will be investigated. The technique will employ a sealed source of 2-5 mCi of Ce139 as the radiation source. Cerium-139 emits characteristic x rays of lanthanum, the energies of which just bracket the absorption edge of iodine. By measuring the relative transmission of the x rays just above and just below the energy of the absorption edge, it is possible to obtain a quantitative estimate of iodine content, that is virtually insensitive to the concentration of other elements in the sample. The system will employ a relatively simple set of calibrated radiation filters to eliminate the necessity for high resolution solid-state detectors for separating the x-ray energies, and will instead use a standard NaI(T1) detector and counting electronics. Preliminary calculations indicate that system sensitivity will be plus or minus 10 micro grams I/ml, in measuring times of a few minutes, in total sample volumes of about 10 ml. Sensitivities of 0.1- 1 mg/ml will be possible in measuring times of a few seconds. The technique is non-destructive and requires no special sample preparation. The major advantage over other absorption-edge transmission or fluorescence techniques is the requirement for a relatively non-hazardous radiation source, and relatively uncomplicated and inexpensive standard radiation detection and counting components. The basic principles can be extended to the determination of iodine concentrations, in vivo, e.g, measurement of stable iodine in the thyroid gland, and this will also be investigated.